International lighting standards require rear bicycle lights to generate a wide angle horizontal beam of light with vertical coverage to make vehicles approaching from the rear or sides aware of the cyclist. At the extremes of the horizontal field the luminous intensities required by these standards are considerably lower than at beam centre but are still of great imoprtance from the point of view of safety. There is often a further requirement for red light to be emitted vertically upwards as a cone that extends in part towards the front of the cycle.
A great many cycle rear lights possess an overall rectangular cross-section, with the long side directed vertically, in which the reflector surrounding the light source has been truncated to the rectangular form of the light. Since it is almost universal for the reflectors to be of paraboloidal form this truncation results only in a loss of optical efficiency rather than a loss of angular coverage. With this type of reflector it is a requirement of the rear lens to extend the angular field sufficiently far to conform with the lighting standards.
In another type of cycle rear light the reflector is designed to contribute at least in part to the generation of the light beam angular distribution rather than to act merely as a collector of light for subsequent angular redistribution by the rear lens. This approach leads to a cycle light which has similar visual properties to a motor car rear light without the very high, and unnecessary, centre field intensity exhibited by a number of cycle rear lights. Should the reflector of a cycle rear light of this latter type be truncated to a rectangular cross-section then there will be a loss of field distribution angle generated by the reflector.
Most commercially available cycle rear lights are designed with an essentially bowl-shaped rear lens moulding which, at least in part, allows the lamp filament to be directly visible at the extremes of horizontal field coverage and in the vertical field. Since the luminous intensity of the lamp is invariably greater than the far field luminous intensities at the extreme angles which are required by the lighting standards, direct viewing of the filament, usually by way of a truncated reflector and the lens moulding, is acceptable.
A second design of cycle rear light lens relies on refraction or reflection within the optical system of the light to generate the required field coverage. Care must be exercised in this approach so that areas of the required field coverage are not omitted or produced at too low a luminous intensity to reach the relevant lighting standards.